4160 and 25KV TX Fusing

[lennic]

Hey there just looking for some info, does anyone have any charts showing fusing for distribution tx's on a 4160 and 25 KV system. All my past experiance has been with a 12470 system. We would always use 1 amp for every 5 KVA could this still apply to these 2 systems. Any help would be great.

[Electriceel]

As a general rule, we figure the rated full load amps on the primary side and take that number times 2.
This will get you plenty of room for overloading of transformers of which all utilities seem to be guilty of this time of year.

[thrasher]

I hate to say it but you really need a fuse chart made out by an Engineer. We work with a couple of Municipals locally that run 2400/4160 volt one uses type KS fuses the other uses type QA fuses. For a 50 KVA you use a 40amp type KS but a 50amp type QA and while I've never checked I would bet it would be a 30amp type T. While electriceel's method works when you're out of town on storm trouble, if you are regularly working a system the system SHOULD have a fuse chart.

[Pootnaigle]

On 2400 xfmrs in a 4160 system we used the same size fuse as the transformer, example 25 Kva 25 amp fuse, 50 Kva 50 amp fuse etc this holds true to about a 100 kva pot. its all according to the companies fusing plan. Many overfuse em intentionally while others under fuse em. Also the curve is different on different types of fuses. some of em melt way faster than others of the same size. Trust me on this but its next to impossible to properly cordinate fuse sizes when using multiple types of fuses.

[Electriceel]

Yep we have a fuse chart but do lineman know how to read.
We have a chart that includes all transformers and sizes, also fusing after reclosures and sectionalizers. I guess we need to include before reclosures also.
Just this past week, we went to a location before reclosures, 35 amp reclosure 2 mile down the line, back at the fused pole someone had installed 20 amp fuses, did not coordinate at all.

Having an engineer do a study for size and types of fuses is great and should be done I think every 10 years, our systems are changing pretty fast.

[compression]

I would think you could just use the KVA / KV. That's how they make the charts anyway.

[GordonBC]

The way we do it is your KVA = Voltage(primary side) x Amps divided by 1000. This works out to A = KVA x 1000 divided by V. to allow for overloading we times the result by 1.5

[thrasher]

The kva/kv method is good for a "rule-of-thumb" but not all fuses act the same. Type N are standard speed, Type T are slow, while Type QA are fast. These are just the three types I have worked on distribution. Then it's further complicated by does the company allow 1.5 times for overload or do they allow 2 times for overload. The bottom line is any decent company has a fuse chart for each voltage they operate. If you are not sure how to read it, ask someone. If no one agrees how to read it then you have just come up with a topic for a safety meeting. Realistically the situation is going to get harder in the future with all the electronic devices starting to hit the line for very large transformers.

[lennic]

Well here is the situtation . I accepted this position in this town. It is a town owned utility 4000 people maybe. The lineman that has been here for 30 some is retiring very very soon. There are no engineers. Basicially the system was put together just so it works. No fuse coordination, no charts, if the fuse blows they just put in the next size until it holds. There is a lot of work to do. They have been using K And KS fuses. So really I could set my own standard on fusing but which style would work best?

[doug]

i was taught trans. X 1000 DIV by the line voltage

ex. 25 kva 25 x 1000 = 25000 DIV line volts 2400 = 10.4166666 so you would use a 10 amp fuse. i was taught to use the single phase volts ex.
4160-2400 12470-7200 24.9-14.4
hope it helps

[md2]

a easy rule of thumb is 100% for 2.4/4, 50% for 7.2/13 and 10% for 19.2/34